(a) Technical Field of the Invention
The present invention relates generally to switches. More particularly, it relates to the design and fabrication of microfabricated electromechanical switches.
(b) Description of Related Art
In communications applications, switches are often designed with semiconductor elements such as transistors or pin diodes. At microwave frequencies, however, these devices suffer from several shortcomings. Pin diodes and transistors typically have an insertion loss greater than 1 dB, which is the loss across the switch when the switch is closed. Transistors operating at microwave frequencies tend to have an isolation value of under 20 dB. This allows a signal to `bleed` across the switch even when the switch is open. Pin diodes and transistors have a limited frequency response and typically only respond to frequencies under 20 GHz. In addition, the insertion losses and isolation values for these switches varies depending on the frequency of the signal passing through the switches. These characteristics make semiconductor transistors and pin diodes a poor choice for switches in microwave applications.
U.S. Pat. No. 5,121,089 issued Jun. 9, 1992 to Larson, and assigned to the assignee of the present invention, discloses a new class of microwave switch--the micro-electro-mechanical (MEM) switch. The MEM switch has a very low insertion loss (less than 0.2 dB at 45 GHz) and a high isolation when open (greater than 30 dB). In addition, the switch has a large frequency response and a large bandwidth compared to semiconductor transistors and pin diodes. These characteristics give the MEM switch the potential to replace traditional narrow-bandwidth PIN diodes and transistor switches in microwave circuits.
The Larson MEM switch utilizes an armature design. One end of a metal armature is affixed to an output line, and the other end of the armature rests above an input line. The armature is electrically isolated from the input line when the switch is in an open position. When a voltage is applied to an electrode below the armature, the armature is pulled downward and contacts the input line. This creates a conducting path between the input line and the output line through the metal armature.
Rockwell International has also developed a MEM switch based on an armature design. The Rockwell switch uses a combination of insulating structural layers and metals as the armature, which increases the mechanical durability of the MEM switch, but the control of the mechanical characteristics, such as internal stress and elastic modulus of the insulating layer, is limited by stoichiometric control of silicon dioxide films. The Rockwell switch uses multiple contact points with flat metal contacts that are likely to have time-varying contact characteristics. In addition, the Rockwell switch is fabricated using an organic polyimide as a sacrificial armature support layer. This leaves organic residue on the switch surfaces after fabrication, which are difficult to remove and adversely affect switch performance and reliability.
Texas Instruments has developed a MEM switch based on a diaphragm configuration. The switch comprises of a flexible membrane supported between two posts. When a voltage is applied between the membrane and an electrode beneath the membrane, the membrane is drawn closer to the electrode by an electrostatic force. The closer together the membrane and electrode, the higher the capacitance between the two. High frequency signals are able to transmit through high capacitances and as such these switches do not need to make actual metallic contact in their "closed" position. High applied voltages are needed, however, to deform the membrane, and isolation characteristics at low frequency are very poor because of the inherent coupling capacitances of the switch structure. In addition, the Texas Instruments switch is highly dependent on membrane stress and on the fabrication process itself, such that the switch is susceptible to material creep and fatigue. Accordingly, there is a need for a MEM switch with a resilient structure and reliable mechanical and electrical contact characteristics.